1. Field of the Invention
This invention relates to electronic flashing devices and more particularly to sensing a charge voltage of a main condenser in a flashing circuit of an electronic flashing device used in photographing with a camera or the like.
2. Related Art Statement
As is well known, in charging a main condenser in an electronic flashing device, such as a strobo device, in order to reduce the charging time, the output voltage of a boosting transformer is often set higher than the actually used voltage, to reduce the charging time by using a steep rise of the charging curve of the main condenser. For example, note the charging characteristic diagram of FIG. 9 in which a charging time T is taken on the abscissa and an output voltage V is taken on the ordinate. When a transformer of a high output voltage is used, and having a charging curve a, the charging time T required to reach the used voltage is short as shown by T.sub.1. When using a transformer having the same output voltage as the used voltage, as shown by charging curve b, the charging time T will be long as represented T.sub.2.
When a transformer of such high output voltage is used, there will be a merit that the charging voltage is short. In such case, unless the charging voltage is controlled, the charging voltage will rise with time to exceed the resisting voltage of the main condenser or the resisting voltage of the flashing discharge tube in which as Xe gas is enclosed. Therefore, when such a transformer is used, it will be necessary to stop the charging operation when the used voltage is reached and it will therefore be necessary to sense the charging voltage of the main condenser. In order to sense the charging voltage, a voltage sensing device such as a neon lamp or Zener diode has generally been used.
As shown in FIG. 10, the conventional voltage sensing circuit using a neon lamp comprises a series circuit of a resistance R.sub.1 neon lamp Ne and resistances R.sub.2 and R.sub.3 connected in parallel with a main condenser C. A voltage rise controlling signal (mentioned as a CHRG signal hereinafter) is set to an "L" level, and serves as a charge starting signal. The output voltage of a battery is boosted by a power source circuit and is coupled through a diode D to a main condenser C, which is charged by the boosted voltage. When the voltage of the main condenser C rises and the charge voltage reaches the lighting voltage of the neon lamp NE, lamp Ne lights to display the completion of charging, transistor Q is switched on by the lighting current flowing through lamp Ne, the signal is inverted by an invertor I, i.e. the CHRG signal changes to an "H" level, representing a charge completion signal, stopping the charging operation.
In the voltage sensing circuit of FIG. 10, when the lighting voltage of the neon lamp Ne is selected to be a desired charge stopping voltage, the charge voltage can be controlled.
In the conventional voltage sensing circuit of FIG. 11, the above-mentioned neon lamp Ne is replaced with a Zener diode TD. The other components of FIG. 11 are the same as the components in FIG. 10. In this case, too, when the charge voltage of the main condenser C rises and the voltage of the condenser C reaches the Zener voltage of the Zener diode TD, the Zener diode TD will conduct, a Zener current will flow, turning transistor Q on. Its signal will be inverted by the invertor I, the CHRG signal changes to the "H" level and the charging operation is stopped.
Further, a voltage sensing means in which the voltage of a main condenser is divided by resistances without using a voltage sensitive device and is judged by a voltage sensing circuit, is known by the Japanese Patent Application Laid Open No. 193131/1990. In this circuit, as shown in FIG. 12, the charge voltage of the main condenser C is divided by resistances R.sub.4 and R.sub.5 and their values are judged by comparators CP.sub.1 and CP.sub.2 so that, when the divided voltage is lower than a preset voltage, to be more fully described, it will be judged that the voltage of the main condenser has not reached the used voltage and a DC/DC converter CO continues to operate but, in a case they become equal to a set voltage, the operation of the DC/DC converter is stopped and the charge to the main condenser is stopped.
Now, in a camera with a strobe built-in, there is adopted a means whereby, when the charge voltage to the main condenser of the strobe is below a full charge voltage sufficient to flash a xenon lamp, such as a flashing discharge tube, or is below a charge voltage lower by several steps than a proper exposure value, release of a shutter will be locked to prevent making a photograph due to a large exposure shortage.
Now, in the above mentioned conventional voltage sensing circuit using a voltage sensing device such as a neon lamp or Zener diode, there is only one voltage detecting level. The charge stopping voltage and the flash permitting voltage must be set at the same value so that, when the release button is pushed, unless the charge stopping voltage is reached, that is, unless a full charge is reached, operation of the flash will not be permitted. Therefore, even if, below a full charge, a charge voltage at which a flash photographing can be made is reached, during the strobe charging, no release will be made and, in the case of a continuous photographing, when a full charge is reached, release will be made. Therefore, there are defects that the continuous photographing time will become long and a shutter chance will be missed.
In the conventional voltage sensing circuit shown in the above mentioned FIG. 12, when the set voltages Vref.sub.1 and Vref.sub.2 of the comparators CP.sub.1 and CP.sub.2 are made different from each other, the charge stopping voltage and flash permitting voltage are separately sensed/but there are defects in that a separate voltage judging circuit is required, the fitting space increases and the cost of the camera also increases.